Development of fuels and fuel sources that may reduce dependence on petroleum has become a massive undertaking for industries having large needs for fuels, such as the aviation industry, for example. Many alternative fuels have been developed, and many remain to be developed, but whenever an alternative fuel is proposed for widespread use, it must be rigorously tested to ensure it meets all necessary specifications. For example, the first generation of synthetic paraffinic kerosene (SPK) alternative fuels was produced via the Fischer-Tropsch (F-T) process. Subsequently, UOP and others developed a second generation process based on the hydrotreating of esters and fatty acids (HEFA) (i.e., seed oil or waste oils and grease) to produce hydrotreated renewable jet (HRJ). The properties of these candidate alternative aviation fuels have been compared to the established standards.
The MIL-DTL-83133G specification for United States Air Force aviation fuel (JP-8) includes 27 separate requirements ranging from physical properties (flash point, heating value, boiling range), corrosion resistance, thermal stability, and production quality (particulate matter, gum content, etc.). This petroleum-based specification has evolved as the knowledge base of the aviation industry has grown over the last hundred years. In contrast, the knowledge base for alternative aviation fuels is much smaller.
New non-petroleum fuels must be subjected to extensive evaluation. Candidate fuels may be evaluated using a multi-step process including an R&D phase that includes the determination of physical and chemical properties, fit-for-purpose characteristics, as well as component rig testing (for operability and durability). These tests may be followed by certification testing where engine and auxiliary-power unit performance are evaluated. It has been found that the low fuel density, low seal swelling, and lack of aromatics of the F-T and HRJ fuels affect aircraft range, fuel gauging and leaking, lubrication, and combustion, making the fuels unsuitable as direct drop-in replacements. However, blends of JP-8 and SPK (from either F-T or HEFA processes) have shown great promise. In these blends, the petroleum-derived fraction provided molecular components that fulfilled a variety of performance requirements that pure SPK could not. Thus, some large-scale operations such as the United States Air Force have certified using a 50/50 mixture of JP-8 and F-T SPK. A second series of fuel certification trials has been successfully conducted using a 50/50 mixture of JP-8 and HRJ.
Next generation fuels are now under development focusing on biobased feedstocks such as lignocellulose and sugar/starches. Biobased fuels have the potential to be cost competitive, sustainable, capable of being produced in significant quantities, and have greenhouse-gas footprints that are lower than petroleum-based fuels. If users want to exploit such potential alternative fuel, it is important to understand the relationship between fuel composition and fit-for-purpose characteristics and engine performance.
The aircraft-engine industry and the Air Force identified a weakness in the current testing of alternative fuels in combustion rigs and engines. The performance of the alternative fuel was compared to a JP-8 baseline, but the JP-8 used was whatever was available. If the alternative fuel performed worse (or better) than the reference, it was not clear how that alternative fuel performance fit into the “experience base” of current jet fuels. The alternative may have been worse than the JP-8 tested, but perhaps well within the “typical” range for that particular property. So, the industry decided that what was needed were reference fuels for testing that would span the range, from “worst case” to “average fuel, to “best case.”
One property area being studied is boiling point range. Whereas current fuels such as F-T SPK and HRJ contain a large range of carbon numbers, generally from C7 to C16, the next generation alternative fuels, such as those produced by alcohol oligomerization or direct fermentation, contain only a few carbon numbers. This narrow carbon-number distribution results in a narrow boiling point range, which might be a concern for engine operability. To determine the effect of a narrow boiling point range on engine operability, it is necessary to select and then produce suitable reference fuels in quantities large enough to support a range of tests, up to and including auxiliary power-unit combustion trials.
In this regard, a “worst case” reference fuel having a narrow boiling point range may be a fuel consisting essentially of alkanes of a single carbon number. Even so, to be amenable to a full range of testing, the reference fuel must be formulated or produced to meet certain specifications of existing fuels such as JP-8. The specifications may include freezing point and/or flash point, for example. Thus, in theory if a test run performed using a reference fuel is successful, it may be reasoned that narrow boiling point range or low flash point in alternative fuels are not likely to represent issues for engine operability.
In view of the above issues, there remain needs for fuel compositions having narrow boiling point ranges and sufficiently low freezing points to enable them to be used as fuels, fuel components, or fuel additives, whether for general use or as references to be compared against existing fuels. Accordingly, further ongoing needs exist for methods that can efficiently produce such fuel compositions in quantities sufficient for large-scale testing operations or general use.